A venous reservoir bag is commonly used in a cardiopulmonary bypass loop. A reservoir bag provides an accumulator function in that it allows for variations in blood volume when cardiopulmonary bypass is conducted with a membrane oxygenator. In addition, the reservoir bag separates bubbles from the venous blood or prime solution circulated in the bypass loop and vents the gas to atmosphere. Gas bubbles may enter the cardiopulmonary loop as a result of air or other gases being in the lines before priming or being entrapped in the venous blood at the operating site.
One prior art reservoir bag has a blood inlet and a blood outlet along the lower edge of the bag and an air vent at the top of the bag. As blood flows through the bag from the inlet to the outlet, gas bubbles rise and are vented through the vent. Unfortunately, prior art bags of this type do not remove all of the gas bubbles from the blood, particularly when blood flow rates are high, the gas bubbles are relatively small and there is a low liquid volume in the bag. Although the oxygenator will remove some of the gas bubbles, the primary gas removal occurs in the venous reservoir bag. It is important that all of the gas be removed because gas emboli in the cardiovascular system of the patient can be fatal.
Siposs U.S. Pat. No. 4,344,777 shows a bubble trap for arterial blood in which a filter element is used to assist in the separation of the gas bubbles from the blood. Although a porous filter element can be quite effective in this separation function, there is a danger that the filter element can become plugged, in which event, blood flow to the patient would be terminated. In an attempt to deal with the clogging problem, the patentee provides a bypass outlet for use in those cases where the filter element is plugged up but system flow must continue. The bypass outlet is connected by a flexible tube to the regular outlet tube, and a clamp normally closes the bypass outlet. This requires that the attendant immediately observe the clogged condition of the filter element and manually open the clamp in order to prevent blood flow back to the patient.
It is also known to provide an elastic, pressure-sensitive valve between an inlet and one outlet of a filter so that undesired pressure buildup at the inlet results in at least some of the liquid to be filtered passing from the inlet directly to the outlet. This filter, which is shown in Shaldon et al U.S. Pat. No. 4,498,990, does not remove gas emboli from blood.